Satellites Discovered Something Bizarre in the 1960s — We Finally Know What It Is
It's called the "ambipolar electric field," and it explains one of the weirdest things our planet's atmosphere does.
Earth has a global magnetic field that’s flinging tiny bits of our atmosphere out into space.
Researcher’s from NASA’s Goddard Space Flight Center recently launched a rocket high into the sky over the Arctic to measure the electrical charge of Earth’s atmosphere. The mission, called Endurance, found a tiny but significant difference in the electric potential (think of potential as the electric version of air pressure) between the air 150 miles up and the air about 477 miles up. The difference is enough to explain something bizarre that satellites first noticed in the 1960s: Streams of electrically-charged particles seemed to flow out into space from Earth’s poles.
NASA atmospheric scientist Glyn Collinson and his colleagues published their work in the journal Nature.
The ambipolar electric field boosts hydrogen ions out into space, fueling a strange phenomenon called the polar wind.
How Earth’s Global Electric Field Works
Just as water in a pipe tends to flow from areas with high pressure to areas of low pressure, differences in electrical potential can push and pull electrically charged particles, like hydrogen ions. The difference, across roughly 300 miles of altitude, is only about half a volt, but that’s enough to levitate positively-charged hydrogen and oxygen ions upward with more than enough force to overcome gravity and launch hydrogen ions into space.
“A half a volt is almost nothing — it’s only about as strong as a watch battery,” says Collinson in a recent statement from NASA. “But that’s just the right amount to explain the polar wind.”
Collinson and his colleagues call the weak, yet mighty, electrical field they’ve discovered the “ambipolar electric field,” because it works in two directions at once: It pulls negatively charged electrons downward and lifts positively-charged ions upward. The ambipolar electric field is as important to how our atmosphere works as gravity and our planet’s magnetic field, even though it creates polar wind across just a few hundred miles around each pole.
When sunlight hits the upper layers of the atmosphere, its energy is enough to knock the electrons off the lazily drifting atoms. That leaves positively-charged ions and negatively-charged electrons floating around up there, and an electric field forms between them.
To understand the effect this subtle electric field has on our planet’s upper atmosphere, picture what happens when you rub an inflated balloon across a cat’s fur. The cat’s fur puffs upward and outward, lifted by the force of negatively-charged electrons repelling each other. Under the influence of the ambipolar field, Earth’s upper atmosphere puffs up, too; ions float higher than they otherwise would. And some of them escape.
“It’s like this conveyor belt, lifting the atmosphere up into space,” says Collinson.
But we’re not in danger of losing all our air, or even most of it the way Mars did in its ancient past. Even with the polar wind blowing at full strength, Earth is losing hydrogen just a little at a time. Meanwhile, things happening on our planet’s surface and deep underground keep pouring new gases into the atmosphere: volcanoes and photosynthesis do most of that work.
This video from NASA explains more about how the ambipolar electric field works.
Where Do We Go From Here?
Scientists have been looking for the ambipolar electric field since the 1960s, when satellites first spotted the polar wind. Now that Collinson and his colleagues have found it, they say it’s time to figure out what role it’s played in the history of our planet. Earth’s atmosphere has changed dramatically several times throughout our planet’s history, and all of those changes would have affected — and been affected by — the polar wind. We’re just not yet sure exactly how.
Understanding Earth’s ambipolar electric field better may also help us fully explain what happened to the atmosphere on Mars, how the atmosphere on Venus became the thick, acidic mess that it is today, and where to look for breathable atmospheres on distant worlds.
“Any planet with an atmosphere should have an ambipolar field,” says Collinson. “Now that we’ve finally measured it, we can begin learning how it’s shaped our planet as well as others over time.
